1. Field of the Invention
The subject invention pertains to a continuous process for the production of high purity polycrystalline silicon in a fluidized bed reactor.
2. Background Art
In a simplified explanation, during fluidized bed deposition of polysilicon in a reactor, a bed of silicon particles is initially introduced, the bed is fluidized by a gas, and heated by a suitable device to the temperature required for the deposition reaction. A silicon containing compound contained in the gas, generally silane SiH4 or halosilanes SiHxXy (Cl, Br, I, F), decomposes at the hot particle surfaces in a pyrolysis reaction with formation of elemental silicon that deposits on the surface of the silicon particles in the fluidized bed and leads to growth in size of the particles. The process can be operated continuously in the steady state if particles that have grown in size are continuously removed as product from the fluidized bed and particles of smaller size, so called silicon seed particles, are fed to the fluidized bed again.
In general, not only the silicon containing compound but also a silicon-free gas, which is referred to hereinafter as a “dilution gas,” is fed to the fluidized bed. Examples of dilution gases are hydrogen, nitrogen and argon. The feeding of the silicon containing compound, by itself or mixed with dilution gas, is referred to below as the “reaction gas.”
The central problem in the fluidized bed deposition of silicon is the fact the silicon containing gaseous compounds react not only at the hot particle surfaces, but also, undesirably, at hot reactor components. This affects in particular the wall of the fluidized bed. In the absence of special measures, a silicon layer deposits on the wall, becomes thicker and thicker over the operating time of the reactor, and thus limits the maximum operating time. This problem is particularly serious because the fluidized bed is generally heated precisely by the wall and a growing silicon layer impairs the heating function, since it represents a thermal insulator. Mechanical stresses on account of the different thermal expansion of wall material and grown silicon layer can also lead to chipping off of parts of the silicon layer or even to the breaking of the fluidized bed wall. Besides deposition on the wall of the fluidized bed, a further problem is deposition of solid silicon on the reactor components which serve for admitting the reaction gas, that is to say the silicon containing gas or gas mixture, into the fluidized bed. Here the wall deposition can lead so far as complete blockage of the inlet.
An elementary factor in fluidized bed deposition is the net heating requirement of the fluidized bed: the amount of heat fed to the reactor via the reactor wall. This has a great influence on the cost of the process. With increasing deposition on the reactor parts, the energy requirement increases and, in the extreme case, the reactor can no longer be heated and has to be shut down. The net heating requirement results for the most part from the difference between fluidized bed temperature and temperature of the gases fed. By contrast, the reaction enthalpy of the gas phase deposition is of secondary importance. The fluidized bed temperature corresponds to the required reaction temperature for the pyrolysis reaction and depends greatly on the type and concentration of the silicon-containing compound.
The major requirement of the polysilicon product of the fluidized bed deposition is the very high purity demanded. In general, the contamination by metals should be less than 100 ppbw, the contamination by the dopants boron and phosphorus should be less than 1000 pptw, and the contamination by carbon should be less than 1000 ppbw.
Fluidized bed processes for the production of silicon granules by solid deposition from the gas phase are known from numerous publications and patents. Various approaches for solving the problems described are mentioned in the literature:
U.S. Pat. No. 3,963,838 describes a process in which the coating on the reactor wall made of quartz chips off continuously from the quartz wall in the process. What is disadvantageous is that the chipped material from the wall has different characteristics than spherical granules, and there is also the risk of quartz glass breakage in the course of the chipping. The granules may likewise contain undesirable quartz as a result of the wall chipping.
JP 2279513 describes a process with a fluidized bed of simple construction. The reaction gas flows in via a distributor plate at the bottom of the fluidized bed. The fluidized bed is heated conventionally via the wall. In order to prevent the bottom of the gas distributor from being blocked by wall deposition, a small amount of HCl is metered into the reaction gas. As a result of a reduction reaction, Si is thus reduced to chlorosilanes in the inlet region of the reaction gas and the wall deposition is thus reduced or prevented in this region. What is disadvantageous about this method, however, is that wall deposition is not prevented in the region of reactor heating, and high thermal losses arise at the bottom of the fluidized bed. Furthermore, in the case of this procedure, silicon granules are taken off in the reaction gas atmosphere and, as a result, have to be additionally purged (inertized).
U.S. Pat. No. 4,992,245 discloses a method for preventing impairment of heating of the fluidized bed by wall deposition, by dividing the fluidized bed into an inner reaction zone and a heating zone enclosing the latter in a ring shaped manner. In this case, the division is effected by a cylindrical tube. The reaction zone is fluidized by reaction gas and the heating zone is flushed with dilution gas. The granules circulate between the heating zone and the reaction zone and thus carry the heat from the heating zone into the reaction region. What is disadvantageous about this process, however, is that the circulating fluidized bed is of very complex construction and can be produced only with very great difficulty from inert materials (quartz or the like). Moreover, the construction cannot prevent the reaction gas from passing into the region of the heating zone and leading to wall deposition there, which in turn, again impairs reactor heating. The gas feeding arrangement is also very complicated in this process.
U.S. Pat. No. 5,374,413 describes a different method. In order that heating of the fluidized bed is not impaired by wall deposition, the fluidized bed is divided vertically or horizontally into a reaction zone and a heating zone. The division is effected by a wall or a cylindrical tube. The heating in the heating zone is effected by microwave, wherein the wall of the reactor is made of quartz and thus transparent to microwaves. What is disadvantageous, however, is that the heat has to be transported by particle and gas convection from the heating zone to the reaction zone. In the case of vertical separation (reaction zone at the top, heating zone at the bottom), a very high bed results, with the risk of changeover to a slugging mode or the formation of very large bubbles in the reaction region. Wall deposition on the components which separate the zones is also discernible in this process. A major disadvantage is the low efficiency of the microwave heating, which is extremely dependent on the form and size of the reactor and is therefore only of little benefit industrially.
DE 19948395 describes the vertical separation of the fluidized bed into a heating zone and an overlying reaction zone. The separation is effected by the feed device for the reaction gas. The heating zone is fluidized only by dilution gas. The heating is effected by a radiant heater in the region of the heating zone, wherein the fluidized bed wall is embodied in such a way that it is largely transparent to the thermal radiation. The product is taken off from the heating zone. A complicated factor in the case of this process implementation is that the heat has to be transported by particle and gas convection from the heating zone to the reaction zone.
U.S. Pat. No. 6,827,786 likewise describes a vertical separation of the fluidized bed into a heating zone and an overlying reaction zone. The separation is effected by virtue of the fact that only dilution gas flows in at the bottom of the fluidized bed and the reaction gas only flows in further above through radially arranged nozzles. The heating is effected via wall heating in the lower region (heating zone). In the reaction region, too, heating can be effected by wall heating. The difference between this process and those of DE 19948395A1 and U.S. Pat. No. 5,374,413 is by virtue of the fact that the dilution gas is fed to the heating zone in pulsating fashion and this pulsation improves the heat transport from the heating zone into the reaction zone. A further heated reaction zone for converting tetrachlorosilane can also be arranged above the reaction zone. What is disadvantageous here, too, is that the heat has to be transported by particle and gas convection (pulsation) from the heating zone to the reaction zone and wall deposition can still take place in the reaction zone, in particular as a result of the (radial) reaction gas feeding near the wall.
All the processes mentioned in the prior art have the disadvantage that wall deposition of silicon takes place at various components of the reactors and as the operational time increases, and the heating capacity decreases as a result of insulation or components necessary for the process are blocked by the deposition. These effects are associated with increased energy costs, negative safety aspects (blockage of the installation) and outage times of the installation as a result of shutdown and mechanical removal of the deposited layer.